Bone mass is balanced by opposing processes of resorption and formation. Insulin-like growth factor-I (IGF-I) is an important bone growth factor that enhances replication of less differentiated bone cells and matrix collagen synthesis by osteoblasts. Parathyroid hormone and prostaglandin E2 increase cAMP levels in osteoblasts, and intermittent administration of either agent stimulates bone formation. These hormones act in part through a process that relies on cAMP dependent protein kinase A, inducing osteoblasts to express IGF-I. The investigator has identified a C/EBP site that is necessary and sufficient for cAMP-dependent IGF-I promoter activation, found C/EBP delta to be the functional trans-acting transcription factor in fetal rat osteoblasts, and also found that estrogen directly suppresses the stimulatory effect of cAMP on C/EBP delta-mediated IGF-I activation, but not on cAMP synthesis. This is estrogen receptor dependent and does not require an identifiable estrogen response elements or AP-1 binding sequence. The proposed studies will examine protein:protein interactions that influence C/EBP mediated IGF-I activation, and the counter-regulatory suppressive effect of estrogen on IGF-I gene activation. The investigator will determine the molecular actions of PGE2 and l7beta-estradiol on C/EBP delta and C/EBP beta expression by characterizing functional cis- and trans-acting elements that regulate their promoter function, and determine the ability of these agents to modulate C/EBP mRNA stability. He will determine the influence of PKA activation in the absence or presence of l7beta-estradiol co-treatment on translocation of C/EBP delta and beta from the cytoplasm to the nucleus and analyze changes in C/EBP phosphorylation under these conditions. Finally, he will determine the physiologic role of C/EBP delta and beta in cAMP activated IGF-I expression, interactions with estrogen, and ultimately in regulating bone mass in C/EBP knockout animals. Bone mass studies will be conducted using peripheral quantitative computer tomography, and will be correlated with basal and activated skeletal IGF-I expression.